In the medical field, a medical video endoscope has been broadly used. The medical video endoscope has an inserting portion to be inserted in a body cavity for observing an organ from the inside of the body cavity, and/or a treating instrument inserting channel through which a treating instrument is inserted to perform various types of treatments for the organ.
In the industrial field, an industrial video endoscope has also been broadly used. The industrial video endoscope can observe and inspect a scratch or a corrosion inside an object, such as a boiler, a turbine, an engine, a chemical plant, or the like.
Particularly, an inserting portion of a video endoscope used for inspecting a jet engine of an aircraft is desired to be thin, so that the inserting portion can easily be inserted in the engine which has a complicated structure.
Generally, the video endoscope is provided at the distal end of its inserting portion with a solid state image pick-up device which functions as an image pick-up means and includes, for example, a charge-coupled device (CCD). A signal cable for transmitting a driving signal of the charge-coupled device is inserted through the inserting portion of the video endoscope.
The signal cable usually includes a bundle of ten-odd signal lines. Namely, the charge-coupled device is connected to the ten-odd signal lines in order to drive the charge-coupled device and to output the image signal of the charge-coupled device. The ten-odd wires constitute the signal cable for transmitting the driving signal and the like of the charge-coupled device. Signals having different frequencies are transmitted respectively to the ten-odd signal lines constituting the signal cable.
Since the signal cable is inserted through a long inserting portion of the video endoscope, the signal cable has a long length proportional to the length of the inserting portion.
When the signal cable is long, it is necessary to stabilize the signal transmitted via the signal cable. Therefore, it has been common to use as the signal line a coaxial cable having a shield structure for preventing the influence of electrical interference and outside noise.
However, since the coaxial cable has the shield structure, the outer size of the coaxial cable is larger than that of a non-shielded signal line which has no shield structure.
When a plurality of coaxial cables are bundled and used as a signal cable of a video endoscope, it is a problem that the outer size of the signal cable becomes considerably large, thereby thickening the inserting portion of the video endoscope through which the signal cable is inserted.
In order to solve this problem, Japanese Laid-Open Patent Application Publication No. 7-184854 describes a video endoscope whose inserting portion can be thin.
In this video endoscope, a coaxial cable which can avoid noise influence is used for transmitting a high-frequency signal, and a non-shielded signal line is used for transmitting a direct current signal. In this way, depending on what kind of signal is transmitted, different types of signal lines are used, thereby minimizing the outer size of the signal cable which includes a bundle of a plurality of signal lines.
However, in the video endoscope disclosed in Japanese Laid-Open Patent Application Publication No. 7-184854, the outer size of the signal cable is limited, depending on the number of the coaxial cables. That is, unless the number of the coaxial cables used is decreased, the signal cable cannot be thinner.
In order to reduce the outer size of the signal cable, all coaxial cables used for the signal cable may merely be replaced with non-shielded signal lines.
However, because signals having various frequencies are transmitted through the signal lines, electrical interference between signals in the signal cable may occur and noise from a signal line may affect another signal line. As a result, the transmission characteristics of the signal cable are deteriorated so that the charge-coupled device will operate erroneously or cannot be driven sufficiently. Thus, the quality of the image picked up by the charge-coupled device will be deteriorated.
Therefore, in Japanese Laid-Open Patent Application Publication No. 7-184854, the minimization of the outer diameter of the inserting portion of the video endoscope has a limit. Consequently, this cannot provide a video endoscope having a thinner inserting portion.